The present invention relates to a lens driving apparatus which performs focus adjustment (hereinafter abbreviated as focusing) by performing lens position adjustment of a master lens when a subject is photographed using a video camera or the like and, more particularly, to a lens driving apparatus which can prevent defocusing during a zooming operation resulting from manual focusing, or defocusing of a photographed image which would otherwise result from a difference between positions during photographing.
Conventionally, a focusing operation of a lens driving apparatus included a video camera or the like, which is disclosed in Japanese Published No. Hei.6-308361, has been known, and this will be described with reference to figures hereinafter. FIG. 8 is a perspective view illustrating a main portion of a conventional lens driving apparatus. FIG. 9 is an expanded cross sectional view illustrating a relationship between a feed screw and a first arm of a driving member in the lens driving apparatus shown in FIG. 8. FIG. 10 is an expanded cross sectional view illustrating a relationship between the feed screw and a second arm of the driving member in the lens driving apparatus shown in FIG. 8. FIG. 11 is an expanded cross sectional view illustrating a combination relationship between the driving member and a lens holding frame in the lens driving apparatus shown in FIG. 8. FIG. 12 is an expanded cross sectional view illustrating a combination and protective relationship between the driving member and the lens holding frame in the lens driving apparatus shown in FIG. 8. FIG. 13 is a cross sectional view of a main section illustrating the conventional lens driving apparatus when the apparatus is faced in a horizontal direction to perform photographing. And, FIG. 14 is a cross sectional view of a main section illustrating the conventional lens driving apparatus during tilt-photographing.
A structure of the conventional lens driving apparatus will be described hereinafter. In FIG. 8, a lens 1, which performs focusing of a subject is fixed to a lens holding frame 2, made of a resin, by performing an operation such as heat-staking. The lens holding frame 2 is held by a first guide pole 3 and a second guide pole 4, which are arranged in a fixed body tube (not shown) so as to be slidable in the direction of a lens optical axis 5. Further, there is provided a groove 2a on an upper part of the lens holding frame 2, which groove connects a driving member 9 to the lens holding frame 2. There is further provided a feed screw 7a in a driving shaft 7 of a motor 6, which motor is attached to the fixed body tube. The driving member 9 made of a resin is slidably held by a third guide pole 10 arranged at a motor mounting body 8 and extending parallel to the driving shaft 7. Furthermore, the driving member 9 is provided with a notch nut 9a which is threadedly engaged with the feed screw 7a as shown in FIG. 9, and is also provided with a first arm 9b having spring characteristics for supporting the feed screw 7a by sandwiching the feed screw.
On the other hand, a second arm 9d is formed on the driving member 9 as shown in FIG. 10, and a gap between the second arm 9d and a thread part of the feed screw 7a is constructed so as to have a smaller gap size than a screw-thread height size of the notch nut 9a. Further, as shown in FIG. 11, at a portion of the driving member 9 that is connected with the lens holding frame 2 there is provided an edge portion 9c having enough rigidity to slidably move the lens holding frame 2, and also having spring characteristics. This edge portion 9c is inserted into the groove 2a in the direction of the arrow, thereby constructing a backlash-free connection in the direction of the lens optical axis 5. Further, as shown in FIG. 12 at the portion of the driving member that is connected with the lens holding frame 2 there is provided a stopper edge portion 9e which does not abut the groove 2a, so that the edge portion 9c having the spring characteristics is not curved to a spring limit due to shock.
In the conventional lens driving apparatus so constructed, an operation thereof will be described hereinafter. The feed screw 7a formed in the driving shaft 7 is inserted into the notch nut 9a of the driving member 9 so as to be pressured and energized by the first arm 9b having the spring characteristics. In this case, since the driving member 9 is slid by the third guide pole 10 to move in the direction of the lens optical axis 5 when the motor 6 rotates, the lens holding frame 2 connected to the driving member 9 is also slid by the first guide pole 3 and the second guide pole 4 in accordance with rotation of the motor 6 to move in the direction of the lens optical axis 5, thereby to obtain focusing of the lens 1.
However, in this construction, the driving member 9 supports the feed screw 7a by sandwiching it through the notch nut 9a and the first arm 9b. Therefore, when the feed screw 7a vibrates during rotation of the motor 6, this vibration is directly transmitted to the driving member 9, thereby causing the driving member to vibrate around the third guide pole 10. Since the driving member 9 and the lens holding frame 2 are connected to each other in the direction of the lens optical axis 5 by the edge portion 9c, having spring characteristics, without backlash, vibration is transmitted to the lens holding frame 2 without being completely absorbed in the edge portion 9c, and therefore, the lens vibrates, whereby, when focusing is performed, an image shakes so that a photographed video becomes hard to view.
Further, even when a tilt-photography is performed by a video camera which is provided with this lens driving apparatus, there is a problem as follows. Specifically, when a focusing operation is performed while photographing is performed in a state where the video camera is horizontal, the lens holding frame 2, which holds the lens 1, is slid by the first guide pole 3 and the second guide pole 4. Also, the driving member 9 is slid by the third guide pole 10, and the lens holding frame 2 and the driving member 9, having respective clearances d, horizontally move in the direction of the lens optical axis 5 to perform focusing as shown in FIG. 13.
When photographing is performed in a slightly tilted state from this state, as shown in FIG. 14, the edge portion 9c having the spring characteristics cannot absorb a slope of the lens holding frame 2, and therefore, the lens holding frame 2 tilts in a self-weight direction of the lens 1 shown by an arrow W3, by a smaller of a clearance D2 maintained between the first guide pole 3 and the lens holding frame 2 and a clearance D3 maintained between the third guide pole 10 and the driving member 9, thereby accordingly generating a tilt of the lens 1 and then a single blur phenomenon of a photographed video.
The present invention is made to solve these problems, and has for its object to provide a lens driving apparatus which faithfully transmits a feed displacement of a motor as feed displacement of a lens, has a stable driving performance, and wherein a photographed video does not vibrate due to a vibration of the motor.
In accordance with a first aspect of the invention, provided is a lens driving apparatus having a lens holding frame which holds a lens and is guided by a first guide pole and a second guide pole to slidably move in a lens optical axis direction. The apparatus further has a driving shaft having a feed screw which is rotated and driven by a motor, and a driving member which has a rack threadedly engaged with the feed screw of the driving shaft at one end and is engaged with the lens holding frame at another end. The driving member is rotatably mounted to a positioning dowel which is provided on the lens holding frame and extends in the lens optical axis direction via a positioning hole provided in the driving member. Also, a spring member, which presses the rack of the driving member against the driving shaft, as well as provides an elastic rotating force in a direction along which the lens holding frame is pressed against the second guide pole, is provided between the lens holding frame and the driving member. A moment in a direction along which the lens holding frame is pressed against the second guide pole by a self-weight of the lens holding frame is added to the elastic rotating force of the spring member. Accordingly, avoided can be a photographed video that vibrates due to a random vibration produced by rotation of the motor. Also, a feed displacement of the motor can be faithfully transmitted as feed displacement of the lens.
In accordance with a second aspect of the invention, in the lens driving apparatus of the first aspect, the driving member is made of a resin material and includes two integral leg parts that are capable of elastic deformation and parallel to each other. The leg parts are respectively provided with positioning holes at their tips. The positioning dowel formed in the lens holding frame has a taper shape such that a diameter of the dowel becomes smaller toward a tip thereof, and a diameter of a bottom part of the positioning dowel is constructed so as to be larger than a diameter of the positioning hole. Accordingly, feed displacement of the motor during photographing can be faithfully transmitted as feed displacement of the lens, and a continually stable driving performance of the lens can be guaranteed.
In accordance with a third aspect of the invention, in the lens driving apparatus of the second aspect, also provided is a stopper which restricts deformation and removal of the driving member, and is formed integrally with the driving member and the lens holding frame. Thereby, when an external force acts on the lens holding frame due to a fall, shock, or the like, the driving member can be prevented from being separated from the lens holding frame.